As electrophotographic developers, a dry developer in which toner components composed of materials containing a colorant and a resin binder are used in a dry state, and a liquid developer in which toner components are dispersed in an insulating liquid have been known.
In a liquid developer, toner particles are dispersed in oil in an insulating liquid, thereby making it possible to form smaller particle sizes as compared to a dry developer. Therefore, high-quality printouts can be obtained surpassing offset printing, so that the liquid developer is suitable for commercial printing applications. However, liquid developers having low-temperature fusing ability of the toner, from the viewpoint of conservation of energy, and having even more excellent storage stability, from the viewpoint of a long-term storage have been desired.
In addition, in the recent years, with the increased awareness of environmental friendliness, an insulating liquid having a low volatility has been used as a dispersion medium for a liquid developer.
Patent Publication 1 discloses, for the purposes of providing a liquid developer being well dispersible, having a high image density, stably giving high-resolution, high-chroma fused images, and having controlled generation of a solvent steam during fusing, a liquid developer for electrostatic photography containing toner particles composed of a colorant and a resin, wherein the toner particles are dispersed in a carrier liquid having a high-resistant dielectric constant, the liquid developer for electrostatic image characterized in that the above carrier liquid contains at least one organic medium selected from (a) a naphthene solvent having an initial boiling point of 150° C. or higher and (b) a monoester formed between an alcohol having 3 or more carbon atoms and a fatty acid having 5 or more carbon atoms. Also, the patent publication discloses a naphthene hydrocarbon usable as the organic medium, such as Exxsol D80, Exxsol D110, and Exxsol D130 hereinabove manufactured by Exxon Mobile Corporation, etc.
Patent Publication 2 discloses a liquid developer comprising at least a resin binder, a colorant, a polymeric dispersant, and a carrier liquid, as a liquid developer having excellent color developing ability and color reproducibility, and having a stable dispersion state of the toner particles in the liquid developer and a stable optical density even with an increase in the number of printouts or printed area, and having a stable optical density without going through any changes in the developer composition for a long period of time. Also, the patent publication discloses that a preferred carrier liquid is a naphthene hydrocarbon such as Exxsol D80, Exxsol D110, or Exxsol D130 hereinabove manufactured by Exxon Mobile Corporation.
Patent Publication 1: Japanese Patent Laid-Open No. 2002-251040
Patent Publication 2: Japanese Patent Laid-Open No. 2013-130791